Work on this long-term project continues. A book chapter that summarized work to-date was recently published: Rothman, R. B., S. Ananthan, and E. J. Bilsky (2009). Novel opioid antagonists with mixed/dual selectivity. In: Opiate Receptors and Antagonists. The Humana Press, Inc. Publishers. Reginald Dean, Edward J. Bilsky and S. Stevens Negus, Editors, p.p. 137-151. In the present reporting period we've focused work in two directions. The first is to develop a cellular system with which to evaluate mixed mu agonist/delta antagonist compounds. For this purpose, we've used CHO cells that cells that co-express mu and delta opioid receptor. Over the last two years, we've made considerable progress in characterizing this model system. Secondly, we've focused efforts on characterizing previously synthesized compounds in various in vitro assays with the goal of wrapping up several manuscripts in preparation. Both of these efforts progress at a measured pace, limited in large part to resource issues.